Machining equipment and assembly equipment often incorporate movable components that position workpieces and/or tool pieces with respect to one another at a processing station. FIG. 1, for example, generally illustrates a processing station 20 having a table 22 for supporting a workpiece (not shown). The table 22 typically moves along at least one axis of a conventional Cartesian X-Y-Z coordinate system to position the workpiece relative to a tool piece (not shown) for processing.
To move the table along one of the X-Y-Z axes, the table 22 can be driven by a lead screw 24 and a lead screw nut 26. The lead screw 24 is typically an elongated rod having an external thread 30, and the lead screw nut 26 typically has a bore with an internal thread 28 engaged with the external thread 30 on the lead screw 24. Rotating the lead screw 24 with respect to the lead screw nut 26 accordingly moves the lead screw nut 26 axially along the longitudinal axis of the lead screw 24. The lead screw nut 26 is typically fixedly attached to the table 22 such that clockwise rotation of the lead screw 24 moves the table 22 in one direction, and counter-clockwise of the lead screw 24 moves the table 22 in the opposite direction.
The lead screw 24 is generally coupled to a motor 32, such as a servo motor, to rotate the lead screw 24. An operator controlling the motor 32 can control the rotational speed and linear displacement of the lead screw 24. Thus, by closely controlling the rotational motion of the lead screw 24, the operator can precisely control the velocity and position of the table 22 relative to a tool piece at a processing station.
One drawback of conventional lead screw assemblies is that there can be a significant amount of play between the external thread 30 of the lead screw 24 and the internal thread 28 of the lead screw nut 26. In many applications, the threads on the lead screw 24 and the lead screw nut 26 wear down so that the thread forms are not in constant contact with each other. As a result, the lead screw 24 typically rotates a number of degrees before the external thread 30 on the lead screw engages the internal thread 28 on the lead screw nut 26 and moves the table 22. This problem is particularly noticeable when the operator changes the direction of the table 22. The play, or lack of positive engagement between the threads 28 and 30, reduces the precision of the equipment 20.
FIGS. 2 and 3 illustrate a two-part, hourglass-shaped nut 32 that addresses this problem. The hourglass-shaped nut 32 includes a first part 34, a second part 36 and a plurality of wedges 38 that drive the first and second parts away from each other to maintain constant contact between the hourglass-shaped nut 32 and the lead screw 24 (FIG. 3).
As illustrated in FIG. 3, the first part 34 has a first threaded hole 40, a conical first surface 42 and a plurality of first tabs 43 projecting from the first surface 42. Similarly, the second part 36 has a second threaded hole 44, a conical second surface 46 and a plurality of second tabs 47 projecting from the second surface 46. When the first and second tabs 43 and 47 are interlocked to axially align the first threaded hole 40 with the second threaded hole 44, the first and second conical surface 42 and 46 form a V-shaped groove around the perimeter of the hourglass-shaped nut 32. The first and second tabs 43 and 47 also prevent relative rotation between the first and second parts 34 and 36.
Each wedge 38 is configured to be closely received within the V-shaped groove. For example, each wedge 38 has a curved external surface 48 that generally follows the perimeters of the first and second parts 34, 36. Each wedge 38 also has a groove 50 in the external surface 48. The hourglass-shaped nut 32 typically has four wedges 38 that fit into the V-shaped groove defined by the first and second conical surfaces 42 and 46 to form a moveable wedge assembly with an annular groove 50.
During operation, the first and second parts 34 and 36 are joined by interlocking the first and second tabs 43 and 47. The assembly of the first and second parts 34 and 36 is then threaded onto the lead screw 24, and the wedges 38 are inserted into the V-shaped groove in contact with the first and second conical surfaces 42 and 46. A ring-shaped, helical spring 52 is then stretched over the hourglass-shaped nut 32 and positioned into the annular groove 50 to draw the wedges 38 into the V-shaped groove. The spring 52 and the wedges 38 accordingly push the first part 34 away from the second part 36 to constantly drive the internal threads of the first and second parts 34 and 36 against the external thread 30 of the lead screw 24.
The hourglass-shaped nut 32, however, may not work well after a period of time because the external thread 30 on the lead screw 24 may not wear evenly. The thread forms of the external thread 30 will generally be thinner along sections that frequently contact the hourglass-shaped nut 32 than those that seldom contact the nut. When the hourglass-shaped nut 32 passes over a highly worn portion of the lead screw 24, spring 52 and wedges 38 drive the first and second parts 34, 36 apart from one another to maintain positive contact with the external thread 30. Yet, when the hourglass-shaped nut 32 is then moved to a lesser-worn portion of the lead screw 24, the spring 52 prevents the wedges 38 from moving radially outward so that the first and second parts 34, 36 can move toward one another to compensate for the thicker thread forms of the external thread 30. The effective pitch of the internal threads of the first and second parts 34, 36, as defined by the relative positions of these parts, accordingly does not mate with the larger, lesser-worn thread forms of the external thread 30. As a result, resistance between the hourglass-shaped nut 32 and the lead screw 24 increases over lesser-worn portions of the lead screw 24 and may even cause the hourglass-shaped nut 32 and the lead screw 24 to seize together.
To release the hourglass-shaped nut 32 from lesser-worn sections of the lead screw 24, the hourglass-shaped nut 32 is generally disassembled to reposition the wedges 38 in the V-shaped groove so that the first and second parts 34, 36 can be moved toward one another. This solution, however, is only temporary because the first and second parts 34, 36 will spread apart from one another over the highly-worn section of the lead screw 24. The only other solution is to replace the lead screw 24, which can be time consuming and adversely affect productivity.